1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus, an image processing apparatus and an image processing method which image a tomogram of a subject according to echo signals of an ultrasonic wave irradiated to the subject, and more particularly, to an ultrasonic diagnostic apparatus, an image processing apparatus and an image processing method, which have a function for image processing to improve contrast visibility of tomograms.
2. Description of the Related Art
An ultrasonic diagnosis is a diagnosis in which heart beat or motions of a fetus can be obtained as a real-time display through a manipulation as simple as placing an ultrasonic probe to the body surface. Furthermore, an ultrasonic diagnosis has no exposure risk different from X-ray imaging or the like. Therefore, in an ultrasonic diagnosis, an examination can be performed repetitively due to its high safety. The foregoing advantages enable the use of an ultrasonic diagnosis in the obstetrics department, home medical care, etc.
Moreover, an ultrasonic diagnosis apparatus can be moved to a bedside, so that an examination can be readily performed at the bedside, owing to its small system size in comparison with other image diagnosis apparatus for X-ray CT (computed tomography) apparatus, MRI (magnetic resonance imaging) apparatus, etc. Although the ultrasonic diagnostic apparatus varies with kinds of functions furnished therewith, the one small enough for an individual to carry around with one hand has been developed.
An intravenous ultrasound contrast medium has been commercialized, and the contrast echo method is thus being performed in recent years. This method aims to evaluate a blood perfusion when examining, for example, the heart or liver by enhancing blood flow signals with the aid of an ultrasound contrast medium injected intravenously. In most of the contrast media, micro bubbles function as the reflection source.
In an above mentioned examination using an ultrasound contrast medium, a very detailed blood vessel structure can be imaged, compared with the ultrasonic Doppler method used as conventional technology. It is expected that blood flow information at the level obtained by this contrast echo method is to be information of great importance for a differential diagnosis such as the degree of progress of the shunt of vessels or regenerative nodules and the differential diagnosis of a diffuse liver disease or a liver cancer.
The basic use of the ultrasonic diagnosis is to observe the morphology and movement of an internal organ from acquired tomographic images. Another important use thereof is to mutually compare the degrees of luminance (contrast ratios of luminance) expressed in the tomographic image, and acquire diagnostic information therefrom. These methods for ultrasonic diagnosis are used not only in ordinary diagnosis in the B mode but also in the recent contrast echo methods, and the observation of tomographic images and the comparison between contrast ratios of luminance are becoming increasingly important elements of diagnosis.
For example, when a tomographic image is diagnosed by the comparison between contrast ratios of luminance, if the intensity of echo signals enhanced by a contrast medium is compared between a disease region such as tumor and a normal parenchyma region, the magnitude relationship between the blood volumes supplied to the respective regions can be known, so that the degree of malignancy of the target area can be diagnosed from the magnitude relationship between these blood volumes.
In another example, in a moving image varying with time, by grasping the temporal variation in the luminance of the tumor region, it is possible to know the characteristics of blood flows flowing into or out of the tumor region, and identify the disease based on a pattern of temporal variation of the luminance.
Regarding above-mentioned ultrasonic diagnosis apparatus, as well as an image diagnostic apparatus such as an X-ray CT apparatus, MRI, and the like, the visibility of a tomographic image during diagnosis constitutes an important element common thereto, and hence, a technique for improving the visibility of tomographic image has been proposed (see, for example, JP-A-H8-7074). This technique divides a tomographic image of X-ray CT image or the like into a background section and a region of interest based on the difference in the pixel value, and allocates luminance gradation of a low luminance to the background section while allocating luminance gradation of a high luminance to the region of interest, whereby the region of interest and the background section can be definitely distinguished from each other. Furthermore, this technique is devised so that a pixel value that is referred to as a threshold value when the region of interest and the background section are divided from each other, can be altered.
In the ultrasonic diagnostic apparatus, as described above, the improvement in the contrast ratio of luminance is an important challenge. To improve the contrast ratio of luminance, it is necessary to enhance basic performance such as an S/N ratio of the apparatus. However, besides such basic performance of the apparatus, illusion (so-called “optical illusion”) brought about by human feeling may constitute a problem with respect to the contrast ratio of luminance.
FIG. 18 is a real image which explains an example case optical illusion is to be a problem on diagnosis of a luminance contrast ratio. FIG. 19 is an illustration image which explains an example case optical illusion is to be a problem on diagnosis of a luminance contrast ratio.
FIGS. 18 and 19 are schematic views each showing a diagnostic image with the region R of interest vertically oriented in the center of the screen. In FIGS. 18 and 19, the region R of interest has a single luminance, but the background B is varied in the luminance depending on a location. As a result, under the influence of the luminance of the background B, the luminance level of the region R of interest is felt as being different at different locations, and in this case of FIGS. 18 and 19, it is felt as being different along the vertical direction. Such a phenomenon that a luminance level is felt to be one different from the actual luminance level thereof is attributable to human optical illusion.
Moving images are also subjected to influence of the optical illusion as in the case of static images. For example, provided the luminance level of the region of interest is constant in time, when the luminance of the background around the region of interest gradually changes toward the higher luminance side, the luminance of the region of interest looks as if it were changing toward the lower luminance side.
The occurrence of such an illusion may cause a risk that diagnostic information is not properly recognized, which can result in a misdiagnosis. The optical illusion, therefore, must be avoided to prevent a misdiagnosis.
However, the conventional visibility improving techniques have been directed to merely altering luminances of the region of interest and background, and no consideration has been given to optical illusion. Therefore, no countermeasures to avoid the optical illusion have been taken. As a result, as described above, in the case where the luminance of the background varies with time, or varies depending on a location, the optical illusion may still occur and incur a misdiagnosis.
On the other hand, as a conventional solution to the optical illusion, a method has been contemplated in which the luminance level of the region of interest is digitalized, histogram-displayed, or graph-displayed, and in which the luminance level subjected to the digitalization, histogram-displaying, or graph-displaying is referred to during diagnosis.
However, this method for digitalizing or graphing the luminance level makes it difficult to observe the overall diagnostic images. Also, because the histograms use values such as the averaged values and the like with respect to the region of interest, they do not suit to the case where minute changes closely associated with the diagnostic images are observed.